This report describes studies on calcium and phosphate metabolism relevant to mineral homeostasis in aging and age-associated osteopenia. The findings summarize investigations on: 1. The mechanism by which the action of parathyroid hormone (PTH) on Na+-Ca2+ exchange in renal cells in blunted in the aged rat. 2. Calcium absorption in isolated duodenal cells: the effects of age and 1,25-dihydroxycholecalciferol (1,25-(OH)2D3), the active hormonal form of vitamin D. 3. Regulation of the synthesis of the Vitamin D hormone. 4. Bone status in the aging rat. 5. Normal human bone cells in culture. 6. Patch-clamp studies in bone cells. 7. Alpha2-Adrenergic hormones antagonized the response of renal cells to parathyroid hormone. 8. Effect of 1,25-(OH)2D3 and age on tubular reabsorption of phosphate. 9. Thyroid hormone regulation of renal cell phosphate transport.